Active bed bio media can be easily used as an effective mechanical filtration media
Even if you have very good active bed bio media filtration you will still need superb mechanical filtration to keep your pond water clear
All too often, the hobby of Koi keeping is misunderstood in the area of filtration. Any good Koi pond filter system will have two key areas that it addresses - mechanical filtration and biological filtration. In essence, mechanical filtration relives the pond of solid debris and detritus such as dead algae, insects, Koi wastes and so forth. In other words, mechanical filtration gets rid of all the stuff that you can see.
Biological filtration is the opposite. It gets rid of all the stuff that you can't see - the poisons that are dissolved in the water of the pond. These poisons are produced by the fish themselves and also by the decay and rotting of the solids that are not adequately filtered out adequately.
It is NOT sufficient to simply collect all the solids in a mechanical filter where they remain in intimate contact with the pond water as it flows over, through and around these collected solids. This is typically what happens inside sand filters and some bubble bead type reactors. High pressure (and hence expensive to run - increased electricity costs are a fact of life for any high pressure system) can be as efficient as mechanical filters.
Depending on the type of bead used in high pressure bead reactors, and the volumes of solid material entering the reactor, will depend on how effective the media is at providing optimum surface area for beneficial bio film.
A good mechanical filter also will develop bio film on it that will help contribute to the biological activity of the overall filtration system. However, it is best practise to regard this is a bonus, and ignore it for purposes of calculating biological filtration requirements.
Good mechanical filters trap all the fine (and large) particulate matter that occur sin your Koi pond. Typically there are two sources of this - 1) green water caused by green algae blooms (which can only be take out by an Ultra Violet light as they are far too small for any mechanical filter to trap) and 2) everything else.
A mechanical filter traps part 2) everything else and allows you to dispose of it quickly and easily. Sand filters are not a mechanism for the disposal part sadly - they are extremely efficient at trapping particles but when it comes to getting those back out of the filter it becomes a nightmare.
A low pressure mechanical filter on the other hand can be extremely easy to clean. There are a number of different ways of achieving this - the details of which we have right here on this site - carry on reading!